It is common to use tube shields in steam boilers and the like to protect the boiler tubing from the environmental conditions existing within the steam boiler. Such shields guard or shelter the boiler tube surface from the effects of excessive corrosion or erosion thereby extending the life of these boiler tubes.
Generally, tube shields are made of a high alloy steel such as austenitic stainless steel. However, non-metallic materials, such as ceramics, have and can be used for tube shielding. Such material is usually more resistant to attack than the alloy used in the construction of the boiler tubes. Alternatively, sometimes tube shields of an alloy similar to that of the boiler tube is used with the intent being to supply sacrificial material that will corrode or erode in preference to the boiler tube itself.
In any event, these tube shields are typically continuous devices shaped approximately like a tube that has been split longitudinally to form in cross-section an arc of about 180.degree. to about 2250.degree.. Such tube shields are placed over the boiler tube and are held in place via a plurality of welded straps that extend around to the back side of the boiler tube. (For the purposes herein, the front side of the boiler tube is the side facing the onrushing furnace gas.) Alternatively, the shields may be applied by simply snapping them over the boiler tube and allowing the natural elasticity of the shield metal to hold them in place.
Sometimes, when the boiler fuel is coal, these tube shields are made to run at metal temperatures higher than the boiler tube they protect. Such running or melting is controlled by the size of the gap or spacing between the boiler tube and the tube shield. For other fuels, such as fuel oil or refuse, the general objective is to keep the shield as cool as possible, such as by minimizing the gap between the cooler boiler tube and the shield.
Unfortunately, these existing shields have ongoing problems with the straps used to secure them to the boiler tube. Such straps oftentimes break, or the welds come loose, due to the extreme conditions existing within the boiler enclosure. Also, due to the continuous nature of these tube shields, they frequently become distorted or buckle while in service. When this occurs, the damaged boiler tube then imposes a significantly higher load or stress upon adjacent straps that can, in turn, cause these adjacent straps to weaken and break. Additionally, such buckled or distorted tube shields lose their convex or arc shape and act to block boiler flue gas flow around and between the array of shielded boiler tubes. This has the potential of compromising boiler thermal performance, and causing flue gas velocity increases (due to constriction flow area) which can further aggravate corrosion or erosion problems. Consequently, when such injury to the tube shield occurs, it is no longer capable of protecting the boiler tube thereby exposing the boiler tube to the full force and effect of the gas flow stream.
Snap-on shields suffer similar problems, either by losing elastic capture due to thermal softening and/or dislodging due to distortions and buckling.
In many cases, damage to the tube shield and/or its mounting hardware occurs due to temperature variances, usually unavoidable, along the length and around the arc of the shield. In other cases, damage can arise due to the mechanical loads incurred from the use and operation of sootblowers. In still other cases, such damage is the result of a difference in the expansion rates between the tube and the shield when such items are heated. In any event, such extreme operating conditions cannot generally be controlled and thus the need for a tube shield that is capable of withstanding these conditions without incurring any damage.
It is thus an object of this invention to provide a tube shield that surrounds and protects the entire boiler tube rather than just a portion of the boiler tube. Another object of this invention is to provide an assured positive shield capture against the tube. It is another object of this invention to provide a tube shield that does not rely upon straps for attachment to the boiler tube. Still another object of this invention is to provide a tube shield that can easily compensate for or accommodate differences in thermal expansion between the tube shield and the boiler tube without buckling or otherwise being damaged. Yet another object of this invention is to provide a unique manner of securing the tube shield to the boiler tube which minimizes the welding required thereby minimizing the welding required during construction and reducing the risk that such welds will be compromised during operation. These and other objects and advantages of this invention will become obvious upon further investigation.